Excel File cannot be accessed

Hi

I upgraded my Excel for Mac 2011 to version 14.3.0 a few days ago and since then, files that I could open now refuse to open.

I am getting the error message as follows

XXX file cannot be accessed. The file may be read-only, or you may be trying to access a read-only location. Or, the server the document is stored on may not be responding.

The files are in the same place they were when they were opening a few days ago, nothing else has changed except this update!

Any ideas?